A pattern type signature can introduce a scoped type
variable. For example

f (xs::[a]) = ys ++ ys
where
ys :: [a]
ys = reverse xs

The pattern (xs::[a]) includes a type signature for xs.
This brings the type variable a into scope; it scopes over
all the patterns and right hand sides for this equation for f.
In particular, it is in scope at the type signature for y.

At ordinary type signatures, such as that for ys, any type variables
mentioned in the type signature that are not in scope are
implicitly universally quantified. (If there are no type variables in
scope, all type variables mentioned in the signature are universally
quantified, which is just as in Haskell 98.) In this case, since a
is in scope, it is not universally quantified, so the type of ys is
the same as that of xs. In Haskell 98 it is not possible to declare
a type for ys; a major benefit of scoped type variables is that
it becomes possible to do so.

Scoped type variables are implemented in both GHC and Hugs. Where the
implementations differ from the specification below, those differences
are noted.

All the type variables mentioned in the patterns for a single
function definition equation, that are not already in scope,
are brought into scope by the patterns. We describe this set as
the type variables bound by the equation.

The type variables thus brought into scope may be mentioned
in ordinary type signatures or pattern type signatures anywhere within
their scope.

In ordinary type signatures, any type variable mentioned in the
signature that is in scope is not universally quantified.

Ordinary type signatures do not bring any new type variables
into scope (except in the type signature itself!). So this is illegal:

f :: a -> a
f x = x::a

It's illegal because a is not in scope in the body of f,
so the ordinary signature x::a is equivalent to x::forall a.a;
and that is an incorrect typing.

There is no implicit universal quantification on pattern type
signatures, nor may one write an explicit forall type in a pattern
type signature. The pattern type signature is a monotype.

The type variables in the head of a class or instance declaration
scope over the methods defined in the where part. For example:

Pattern type signatures are completely orthogonal to ordinary, separate
type signatures. The two can be used independently or together. There is
no scoping associated with the names of the type variables in a separate type signature.

f :: [a] -> [a]
f (xs::[b]) = reverse xs

The function must be polymorphic in the type variables
bound by all its equations. Operationally, the type variables bound
by one equation must not:

Be unified with a type (such as Int, or [a]).

Be unified with a type variable free in the environment.

Be unified with each other. (They may unify with the type variables
bound by another equation for the same function, of course.)

A pattern type signature can be on an arbitrary sub-pattern, not
just on a variable:

f ((x,y)::(a,b)) = (y,x) :: (b,a)

Pattern type signatures, including the result part, can be used
in lambda abstractions:

(\ (x::a, y) :: a -> x)

Type variables bound by these patterns must be polymorphic in
the sense defined above.
For example:

f1 (x::c) = f1 x -- ok
f2 = \(x::c) -> f2 x -- not ok

Here, f1 is OK, but f2 is not, because c gets unified
with a type variable free in the environment, in this
case, the type of f2, which is in the environment when
the lambda abstraction is checked.

Pattern type signatures, including the result part, can be used
in case expressions:

case e of { (x::a, y) :: a -> x }

The pattern-bound type variables must, as usual,
be polymorphic in the following sense: each case alternative,
considered as a lambda abstraction, must be polymorphic.
Thus this is OK:

case (True,False) of { (x::a, y) -> x }

Even though the context is that of a pair of booleans,
the alternative itself is polymorphic. Of course, it is
also OK to say:

case (True,False) of { (x::Bool, y) -> x }

To avoid ambiguity, the type after the “::” in a result
pattern signature on a lambda or case must be atomic (i.e. a single
token or a parenthesised type of some sort). To see why,
consider how one would parse this:

\ x :: a -> b -> x

Pattern type signatures that bind new type variables
may not be used in pattern bindings at all.
So this is illegal: